1. Technical Field
The present invention is concerned with selecting optimal ink sets for an Expanded Color Gamut (ECG) printing process.
2. Background Information
Traditionally in printing, three or four inks have been used in printing apparatus. A three-ink printing device uses cyan, magenta and yellow inks (CMY). A four-ink printer adds black in to the basic CMY system to achieve deeper shadow tones and this is known as a CMYK system.
This combination of four inks can be used to produce other colors, for example, red, orange, green or blue/violet by overprinting some of the CMYK inks on top of each other.
Whilst this combination of inks has been toured to be effective in producing a wide color range, or “color gamut”, the gamut of colors available to a four-ink printer is restricted by the properties of the four color inks.
As Desktop Color Publishing Systems become more ubiquitous, 4-colour CMYK (cyan, magenta, yellow, black) printing has become a commodity item. Consequently the technical community is being driven by the high-end color market to develop new printing technologies to differentiate and add value to their product. The intent is to produce images of superior color, tone and detail than is obtainable with traditional processes. One approach to obtaining more colorful images is by employing more than 4 inks as process inks; in short, extra-quaternary printing processes.
To meet the demands of very high quality printing, for example in packaging, systems have been developed that add additional color inks (process inks) such as orange, green and violet or red, green and blue to the base set of inks. For example, the system addresses the problems of available cyan, magenta and yellow colorants failing to render bright and saturated orange and green colors by using additional colorants that specifically target blue, orange and green portions of the visible spectrum. While this method is expensive, since it requires the use of additional printing stations to print these additional colorants, the solution is common in the packaging industry, where bright and saturated colors support the sale of a product.
The addition of colors to the traditional CMYK system is known as the Expanded Color Gamut (ECG) printing process. This process is sometimes also known as High Fidelity (Hi-Fi) color printing or N-color printing.
Augmenting a set of CMYK inks with additional inks increases the gamut of printable colors. In any printing process, colors tend to become darker as more ink is laid down on paper because of the subtractive interaction of ink and incident light. The lighter hues of red, green and blue are consequently difficult to achieve with the traditional set of CMYK primary inks since they are produced from 2-ink overprints of the primaries. These lighter hues can be obtained by single inks properly formulated to the desired hue. Additional inks tend to increase the color gamut in the darker tonal regions as well. Their presence chromatically expands the entire gamut up and down the tonal range. The quarter tones and 3 quarter tones are especially augmented.
The additional color inks are pre-mixed. Variations exist, though, in different batches of, e.g., orange, green or violet color. For example, one manufacturer's “orange” may be slightly different from another manufacturers “orange”. Often, therefore, several possible inks are available to produce, effectively, the same additional color and a selection of the optimum ink sets has to be made.
Selection of these process-inks is crucial because it will affect the color gamut of the printing system resulting in success or failure of the ECG printing implementation.
Typically in packaging, the ECG printing is aimed at replacing the spot colors (and the special inks associated with these spot colors) with a fixed set of process inks. There are several inks available as candidates for process inks to choose from, particularly for Orange, Green and Violet. It is difficult to select the best ink-set from these available inks. For each possible combination of ink-set, it is necessary to find out the color gamut, which enables the coverage of spot colors to be checked, i.e. how many of the spot colors can be achieved and replaced by using the given combination of ink-set. It is not feasible to characterize all combinations to calculate a color gamut for each potential ink-set.
It is not practical to derive gamuts of all potential combinations of ink-sets for the ECG printing press. For example, if there are 200 ink-candidates for three process-inks (Orange, Green and Violet), then there are more than 1 million potential combinations of ink-sets for the ECG printing. Traditionally, it would be necessary to perform more than 1 million press-trials in order to derive a color gamut for each combination.
This traditional method requires long “make-ready times” (wash-up), long down times for the printing press, significant material wastage, and requires a large supply of special inks. The present invention aims to simplify selecting optimum ink sets for the expanded color gamut (ECG) printing process without having to print color test charts or overprints of inks on the printing press or printing device. The invention enables selection of an optimum ink set to replace a maximum number of spot colors with a set of, for example, seven inks.